The Peacemaker is the gun that originally belonged to Wyatt Earp.
The Peacemaker is the only weapon that can kill revenants. Once a revenant is shot by the Peacemaker, they will be dragged into hell via a fiery portal in the ground. It has been shown that Revenants cannot touch the Peacemaker without experiencing extreme pain.
According to the Earp family mythology, the Peacemaker can only be wielded by the Earp heir, currently Wynonna. It has been shown that two heirs can wield it interchangeably, as Willa would grab Peacemaker from Wynonna and fire it with no problems. When Agent Dolls attempted to fire the Peacemaker, nothing happened. However, Waverly once claimed that her research gave her no reason to believe she couldn't break the Earp Curse, which suggests it may be possible for her to use the Peacemaker.
After Willa shoots a revenant with Peacemaker, Wynonna realizes who she is. Two days after, Wynonna attempts to use Peacemaker to kill a revenant but it just "flickered and jammed". Later on, Willa attempts to kill Wynonna but the gun refuses to fire.
The Peacemaker is Colt Buntline Special, a six-chamber gun with a long barrel. The gun does not shoot straight, making it nearly impossible to use at long distances. On the inside of the barrel are various symbols which are probably responsible for its powers.